The term “biodiesel” refers to a diesel-equivalent, processed fuel derived from biological sources, such as vegetable oils and/or animal fats. Biodiesel is a processed fuel that can be readily used in vehicles with diesel engines. Biodiesel can be used in pure form or may be blended with petroleum diesel at any concentration in most modern diesel engines. The use of biodiesel offers many advantages. For example, it has been estimated that biodiesel reduces emissions of carbon monoxide (CO) by approximately 50% and carbon dioxide by 78% on a net lifecycle basis because the carbon in biodiesel emissions is recycled from carbon that was already in the atmosphere, rather than being fossil carbon from petroleum that was previously sequestered in the earth's crust. In addition, biodiesel is generally biodegradable and non-toxic.
Biodiesel typically includes alkyl esters of fatty acids. The alkyl group is commonly methyl, ethyl, propyl, or isopropyl, although it can also include higher numbers of carbon atoms. Alkyl esters can be produced from fats or oils via catalyzed or uncatalyzed reactions. Catalyzed reactions for producing alkyl esters can be categorized as either homogeneous or heterogeneous catalyst reactions. The definition of homogeneous and heterogeneous is from the perspective of the catalyst. In a homogeneous reaction, the catalyst is completely dissolved in one of the feed stocks, such as the alcohol, whereas in a heterogeneous reaction the catalyst forms a mixture (it is not dissolved completely) with the total reaction mixture. Reactions for producing biodiesel can specifically include: 1) direct base catalyzed transesterification of the oil or fat with an alcohol; 2) direct acid catalyzed transesterification of the oil or fat with an alcohol; and 3) initial conversion of the oil triglycerides to fatty acids and then to alkyl esters via acid catalyzed esterification.
The direct base catalyzed method, with the base dissolved within the reactants (homogenous catalyst), is currently favored for industrial production of alkyl esters because the underlying reaction is generally faster than some other methods. In the base-catalyzed method, triglycerides from a fat or oil (such as soybean oil) are reacted with a short chain alcohol such as methanol or ethanol in the presence of an alkaline catalyst. The alkaline catalyst is usually sodium or potassium hydroxide that has been dissolved in the short chain alcohol. The short chain alcohol is generally added in a molar excess (e.g., greater than 3:1 of alcohol:triglyceride) to promote conversion of the lipid to biodiesel. The direct base homogenous catalyzed method is commonly conducted in a batch processing mode where the reactants are mixed together in a batch, allowed to react and then the biodiesel is harvested from the resulting mixture.
However, a need remains for methods and apparatus for producing alkyl esters that can be applied in an economical biodiesel production system.